Investigating the effect of BT direction on W source-to-core pathways during the SAS-VW campaign on DIII-D

Experiments using the V-shaped closed slot tungsten (W) coated SAS-VW divertor in DIII-D studied the effects of the B_T direction on core contamination of eroded tungsten from a closed slot divertor configuration. Core W content is inferred using soft-X ray tomography (SXR) and vacuum ultraviolet spectroscopy (SPRED), while W divertor erosion is inferred from visible spectroscopy of W emission (400.9 nm) measured by in-slot filterscopes (filtered photo-multipliers). Post-mortem analysis from the campaign discovered tile misalignment leading to suspected pronounced leading-edge erosion in the unfavorable B_T direction (ion $\overrightarrow{B}\times \nabla \overrightarrow{B}$ drift away from divertor) likely not captured by diagnostics. However, empirical findings show up to ∼ 2-3x larger core contamination in the favorable B_T direction even considering no additional W erosion from leading edges. A “source-to-core efficiency factor” is derived to estimate the effects of leading-edge erosion and compare W contamination for two pairs of H-mode discharges in opposite B_T directions. While having differing absolute parameters, similar core impurity density gradients suggest comparable core impurity transport. These results show that favorable B_T may have stronger source-to-core pathways for W impurities sourced from the outer divertor region. Possible explanations could include the effects of $\overrightarrow{E}\times \overrightarrow{B}$ drifts on W transport in the scrape-off-layer (SOL) as well as previously determined fast SOL inner target directed flows in favorable B_T.